164 me. j. b. scriv^nor on the [May 1903, 



stand out as a ridge, a fact which leads one to suppose that the 

 pampas here represent a plain of marine denudation of recent 

 date. Nor is this the only unexpected feature presented, for 

 throughout its length the dyke is cut by a longitudinal canadon, 

 roughly 200 yards broad, in which is situated the estancia of 

 Mr. Langley, an English settler who has taken advantage of the 

 perfection of the divisional planes in the quartz-porphyry to quarry 

 it for use in Buenos Aires as paving-stone. The alteration of the 

 ' country ' is very marked, and results in a hard red rock with 

 occasional grains of quartz. 



At Trelew a fine-grained, acid intrusive rock is being quarried 

 for local use, as also is the case at Camerones Bay. 



In the valley of the Chico de Chubut two intrusive masses were 

 noted. One, north of Malespina, forming a boss over 40 feet high, 

 stands by itself in the alluvial flat : it also is a quartz-porphyry. 

 Another, some 3 miles distant to the south-west, forms a bold cliff 

 close to the river. The latter of these two masses is remarkable 

 for being intensely contorted, and broken by numerous minute faults. 

 The axes of contortion and the faults trend north-west and south- 

 east; and there is also a well-defined series of joints trending 

 north-east and south-west. The rock is composed of folia of quartz, 

 felspar, and a purple material resembling the groundmass of some of 

 the porphyry-pebbles in the Tehuelche Pebble-Bed ; so that there 

 is strong reason to believe it to be similar to a quartz-porphyry 

 in composition. Unfortunately the ' country ' in the immediate 

 vicinity of the bosses is covered by alluvium, therefore it could 

 not be ascertained whether it is metamorphosed or not. If the 

 two bosses are of the same age, it is strange that one should be 

 undisturbed and the other contorted. The mass is too big to be 

 regarded as the marginal portion of a large boss, where a structure 

 approaching this (but for the faults) might conceivably be pro- 

 duced. At Port St. Helena the intrusion is undoubtedly later than 

 the Tertiary deposits, but this mass must be earlier, since no one 

 has noted in the sediments of the Chico-de- Chubut Valley any dis- 

 turbances sufficient to produce the degree of contortion observable 

 in the porphyry. 



Acid intrusive masses occur on the shores of Lake Buenos Aires, 

 and the moraines in that region show that others must occur in the 

 Cordilleras overlooking the lake. Intrusive masses are also met with 

 in the southern part of Patagonia. 



The specimens of igneous rocks selected from the moraines of 

 Lake Buenos Aires for slicing represent the more commonly-occurring 

 types, and, since they present no important details, it will suffice 

 merely to enumerate them, as follows : 



Biotite-granite, with abundant micro- j Quartz-porphyry. 



perthite, and in one specimen a Rhyolites, with characteristic struc- 



little tourmaline. tures. 



Hornblende-granite. Trachytes. 



Quartz-mica-diorite, with abundant j Olivine-dolerite, with ophitic struc- 



apatite. ture. 



Gabbro. Olivine-basalt. 



Hornblende-picrite. Acid tuffs. 



