Igneous Structures

The most striking thing in connection with the structure is the remarkable uniformity of the sills and their close resemblance on that account, after weathering, to the fragmental beds with which they are associated. The small amount of metamorphosing influence that they seem to have had, also adds to the difficulty. In some cases, however, a transgressive intrusion has disturbed the adjacent beds a great deal in a mechanical way.

In the average case, it is judged that the intruded magma has neither penetrated the materials of the adjacent beds to a noticeable amount, nor

FIG. 11. Shales and ash beds cut by a large irregular dike and sill

The dike is shown at the location of the standing figure and the sill extends upward to the left between the plainly bedded layers. Both the dike and the sill are crowded with fragmental material to an extent that makes the intrusive have more the appearance of a volcanic fragmental than a true intrusive.

has it absorbed or incorporated a great deal of such material. But in a few cases where structural relations were indisputable it was equally clear that the intrusives, both transgressive and concordant, were literally choked with foreign fragmental matter, making them resemble the real tuffs so closely that it is doubtful whether the difference would have been detected except for the clearness of the intrusive relation. Such occurrences suggest that there may well be many other apparently fragmental interbeds that are in reality fragment-clogged intrusives. On ac

count of the great abundance of the fragmental matter, it does not seem possible that these intrusive masses could penetrate in that condition to some of the positions where we now find them. It is more likely that a rather fluid magma has penetrated some unusually porous fragmental bed forming a matrix for it, perhaps also spreading it somewhat, and then in breaking across to another bed, in some cases it was still mobile enough to drag the mixture along into the larger transgressive structures. This idea is somewhat supported by the finding of a conglomerate bed, near K-86 on the military road west of Aibonito, impregnated with an igneous matrix in essentially this same manner. In addition to these

forms, there are numerous larger intrusive masses, the largest of which deserve a special name. I see no objection to calling them bosses. The two largest occur, one between Caguas and the Caribbean sea toward the south and southeast, and the other between Jayuya and Utuado on the north side of the divide.

Volcanic Vent Complexes

A special igneous structure that has not been referred to except incidentally is that composite of disturbed structures which represents the location of old volcanic vents. They are essentially a complex of irregular intrusive units cutting and including masses and aggregates of various fragmental and sedimentary types in a mixed relation. In the clearest cases, such a complex suddenly takes the place of a formation of apparent promise of continuity such as a series of sedimentary beds, and after an interval these beds are again found continuing as before. For example, the Coamo limestone formation is abruptly cut off and its place is taken for a mile or more by one of these igneous complexes, the limestone continuing on the other side again. The Jacaguas reservoir, just above Juana Diaz, lies in one of these old volcanic-vent complexes, occupying, however, only a portion of the area. Another such case is represented by the complex cutting the great conglomerate beds on the military road at about K-87-88 west of Aibonito. Another is believed to be represented by the very striking basin-like area crossed by the road. between San German and Hormigueros. This one is now represented by a very smooth plain five or six miles across surrounded on all sides by more hilly country. The same conditions are undoubtedly indicated by the extremely complex structures seen on the Descalabrado river two miles below, south of the military road. Some of these mark the sites of ancient craters, clogged or choked with fragmental and intruding materials.

Folding

Most of the rock formations representing original bedded types have been more or less tilted or otherwise do not now have their original attitude. Those belonging to the younger series, the Tertiary limestones and shales on the north side of the island, are comparatively little disturbed, and in some cases do not have a very different dip in spite of the fact that they have changed very much in position with respect to sea level. On the south side of the island, however, even these later beds are in many places tilted at a higher angle than they had in the beginning

and occasionally show high angles and even gentle folds. This condition may be seen on the Jacaguas River, near Juana Diaz, better than at most places, but similar conditions are indicated by the relations at certain points farther west. This condition on the south side of the island indicates more extensive and more violent dynamic disturbance on this side, which is further supported by the presence of faults cutting and affecting the Tertiary series on its present inner margin.

The older series, the pre-Tertiary, is still more profoundly affected and, in almost all districts, shale and ash beds may be found standing at high angles, in many cases practically vertical, and in occasional instances

crumpled and overturned in a most complicated manner. Minor fold structures of this kind are especially prominent in the higher ranges, for example, along the Jayuya road along the divide opposite station K-24 of the Ponce-Arecibo road. At such a place may be found as complicated structural features of this kind as is usually present in any folded mountain region. The high angle at which such beds stand at many other points leads to the belief that similar complexities characterize a great many of the districts occupying the higher mountain portion of the island as well as some of the marginal areas, but the great amount of erosion and the very limited exposures at most points tend to obscure some of these details. The complex way in which the igneous intrusive masses cut all of these formations also tends to obscure and modify and further complicate the simpler fold structures so that it is not always possible to properly credit the disturbed attitude. On account also of the fact that the total quantity of injected or intruded materials, including dikes, sills and bosses, is exceedingly large and must have caused extensive disturbances by reason of the displacement produced by the occupation by these intrusive masses, it is likely that much of the observed abnormal attitude of the bedded rocks may be due to this cause rather than to regional folding of a simpler sort. It is fair to say, however, that a sufficient amount of data is not yet available to draw general conclusions as to the meaning of the fold structures in Porto Rico. The striking thing is that all of the older formations are disturbed and that their position and attitude, even along the margins of the island, indicate that the region affected by these movements was more extensive than the present land area.

Faulting

There are many evidences of faulting on a small scale, in some of which the displacements can be measured. But in most cases the direct evidence lies chiefly in the existence of crush zones, slickensided walls and abrupt changes of rock type; there is no opportunity, on account of the general rock complexity, to secure quantitative data. Judging from the difficulty in tracing certain formations between districts where they have been identified, it is probable that there are occasional faults of large displacement. Numerous crush zones were seen on the Comerio road especially, but in this case also no system was discernible from the few measurements available.

The most prominent fault, in its effect upon present features, is the one now marking the inner margin of the younger series of chalky limestones and shales constituting the coastal belt along the south side of the

island from Juana Diaz past Ponce at a short distance to the north, crossing the Ponce-Arecibo road at K-4.8, and thence westward, crossing the Ponce-Penuelas road at K-10. This is the only large fault actually observed that is necessarily of recent age, although a few others are inferred. It must be of very late Tertiary age, because the chalky Ponce beds are abruptly cut off by it. The older rocks of the pre-Tertiary are lifted with respect to the younger series forming the present coastal margin wherever this fault has been seen. It has been traced by us from Juana Diaz to the vicinity of Penelas, a distance of about 12 miles. What becomes of

it at either end is not yet determined, but it is believed to extend much farther in both directions.

The physiographic habit of the island as a whole tends to support the view that the fundamental structural form is that of a large fault block, with the principal fault displacement and uplift along or near the southerly margin, tilting the whole mass gently northward. If this disturbance took place, as seems to be indicated by the fault described, in very late Tertiary time, accompanying the emergence from the sea, it would account for the abnormalities of Tertiary rock distribution as well as the unsymmetrical position of the main drainage divide. In any case, however, the fault block structure is a very late development and is superimposed on the other more complex and older structures of the mass.