

OF CENTRAL AND EASTERN CORNWALL. 473 



composed of reticulated felspar crystals, of which the majority are 

 triclinic, although monoclinic felspar is also present. Many of the 

 crystals of felspar are about 1*5 millim. in length, and are not 

 unfrequently more or less curved. With the felspar are associated 

 crystals and crystalline patches of augite, as well as numerous 

 pseudomorphs after that mineral : the latter are usually composed 

 of felted hornblendic belonites ; but viridite in small quantity is 

 also present. The augitic crystals are generally converted near 

 their edges into feathery hornblende; and their structure is some- 

 times partially destroyed by patches of viridite. This rock likewise 

 contains numerous hexagonal prisms of apatite, and small crystals 

 of magnetite which are apparently unaltered ; no grains of quartz, 

 such as are occasionally met with in specimens from the shores of 

 Mount's Bay, were observed in any of the six sections of this rock 

 which were examined. A small quantity of the bright blue mineral 

 (indicolite), mentioned in a former paper as occurring in the rock from 

 Terrace Quarry, was found in one of the sections. 



The comparatively coarse-grained variety, in which felspar is 

 readily distinguished by the naked eye, is seen under the micro- 

 scope to be composed largely of but slightly altered crystals of pla- 

 gioclase, although monoclinic felspar is also present. From lithia 

 having been found in this rock it is probable that some portion of the 

 monoclinic mineral may be spodumene *. 



No unaltered augitic mineral was detected in any of the sections 

 of this variety of rock, although hornblendic pseudomorphs after 

 that mineral are numerous ; in other respects the coarsely crystal- 

 line rock resembles the fine-grained variety. 



The foregoing examination of thin sections of this rock con- 

 clusively shows that it is not an altered diorite, as was formerly sup- 

 posed, but that, on the contrary, it is a modified dolerite of the class 

 abundantly occurring in the neighbourhood of Penzance. 



A quarry which was opened many years since on a somewhat 

 similar band at Blowing-House Hill, immediately west of the town 

 of St. Austell, is now full of accumulated rubbish ; but if fresh speci- 

 mens of rock were obtainable from this locality, it would probably 

 be found nearly similar in composition to that above described. 



At Duporth, two miles east of St. Austell, is an outcrop of eruptive 

 rock in the sea-cliff bounding the property of Mr. Gr. Freeth. This 

 rock, which consists of a greyish matrix enclosing numerous angular 

 white spots, is soft, and somewhat unctuous to the touch, while the 

 general appearance of the mass is that of an aggregation of more or 

 less rounded boulders cemented together by a mixture of an 

 asbestus-like mineral and hydrated ferric oxide. 



On closer examination it becomes evident that this peculiar 

 structure of the mass is due to the circumstance that the rock, 

 which is highly magnesian, has undergone decomposition along 



* When it is remembered that the atomic weight of lithium is only 7, while 

 that of potassium is 39"1, it becomes evident that the amount of lithia found is 

 of more importance than would at first sight appear. Traces of lithia were 

 found in various rocks of this class in which its amount was not estimated. 



