E. Hoive — Geology of the Isthmus of Panama. 233 



Basic Intrusives. 



The last phase of eruptive activity of which there is evidence 

 in the Canal Zone is represented by numerous basic intrusives 

 that occur in great abundance in the central and southern 

 parts of the isthmus. To tjie north, on the Atlantic coast, 

 they are exposed in the vicinity of Porto Bello, and about 

 eighteen miles east of Colon. Between Gorgon a and Pedro 

 Miguel most of the hills are composed of the basic rocks. In 

 all observed occurrences the rocks are intrusive in the older 

 formations and most commonly occur as large stock-like 

 masses ; dikes are numerous but no surface flows have been 

 found. The rocks that have been collectively referred to as 

 basic intrusives are pyroxene-andesites or basalts. All are 

 notable for the calcic character of the feldspar, which is fre- 

 quently bytownite or anorthite. Augite and magnetite are 

 abundant and many of the rocks contain considerable ortho- 

 rhombic pyroxene. The majority of the rocks are pyroxene- 

 andesites, but basalts are common and in one or two instances 

 were found to contain much olivine ; practically all have more 

 or less glass in the ground mass and some are vesicular. 

 Although undoubtedly related to the andesites from which the 

 Obispo breccias were derived, none of the Obispo rocks shows 

 the extremely basic character of many of the later intrusives. 



At many places in the interior the basic intrusives may be 

 seen cutting the older sedimentary rocks, while the relations 

 near Panama seem to indicate that the andesite of Sosa Hill 

 has been intruded into the rhyolitic tuffs of An con. So far 

 no basic rocks have been observed to cut the Monkey Hill for- 

 mation, but the large masses of pyroxene-andesite at Porto 

 Bello are probably intrusive in these sediments although no 

 contacts have been observed. The very uniform lithologic 

 character of the Monkey Hill beds, and the lack of any 

 observed unconformity indicating decided crustal movements 

 such as must have accompanied the intrusion of the andesites, 

 would seem to show, in the absence of any more direct evi- 

 dence, that the period of basic intrusions followed the 

 Monkey Hill epoch and may have been the immediate cause 

 of the termination of that long period of quiet sedimentation. 



Distribution of the Mocks. 



Unfortunately little is known of the areal distribution of 

 the various formations that have been described. The section 

 exposed along the canal line is fairly complete and in places 

 it has been possible to trace certain of the formations a few 

 miles beyond the boundaries of the Canal Zone, but even if 

 greater areas had been examined it is doubtful if any more 



Am. Jour Sci. — Fourth Series, Vol. XXYI, No. 153. — September, 1908. 

 17 



