218 JOURNAL OF THE PLYMOUTH INSTITUTION. 
of Triassic breccia, the cementing material of which is partly 
calcedonic, which I found at Drake’s Island. 
Inter alia, the variety of rocks at Drake’s Island is something 
remarkable in so limited a space. Mr. Prideaux stated that it 
was dunstone or greywacke, which terms were used by him 
interchangeably, with a little elasticity, to indicate trappean rocks. 
Dunstone is the ordinary local name for our volcanic series, but 
greywacke was commonly employed by our older geologists as the 
name of a system, and was occasionally adopted by Mr. Prideaux 
in that sense. In addition, however, to the volcanic rocks which 
form the bulk of the island, and which range from the ordinary 
lava type to schistose ashes and slates charged with lapilli, we 
find limestone and sandstone and the breccia already named. 
Limestones.—Our limestone rocks are rather organic than sedi- 
mentary—in the main a consolidated coral reef. The animal 
origin of this is readily apparent; and surrounding it are more 
homogeneous calcareous masses, produced by the waste of the 
reef and the redeposition of its particles, and frequently en- 
closing large quantities of shells. This limestone has undergone 
much change, and is now highly crystalline. It is jointed with 
considerable regularity, and on a definite system; is traversed 
by a number of caverns; and is in parts markedly dolomitie, 
containing considerable quantities of magnesia. There appears 
reason to believe that the black hue, characteristic of some 
localities, was originally due to the presence of carbonaceous 
matter; and carbonaceous films have been noted. It yields some 
of the most beautiful marbles in the kingdom. 
The bulk of the Plymouth limestone forms a band stretching 
along the northern shore of the Sound, eastward from the 
Hamoaze at Devonport, to a little beyond Gore. There are 
several isolated masses, the largest that of Yealmpton, and others 
of less importance at Goosewell, Craza Mill, and Hessenford, 
besides such intermittent beds as occur between Batten and 
Bovisand. The patch at Mount Edgcumbe, at what is usually 
called Cremyll, is really a portion of the Plymouth band. 
Mention has already been made of the “hard-head,” as a kind 
of bastard rock between sandstone and limestone. The microscope 
shows the quartz grains in a fine calcareous matrix. There is 
also a passage observable on the verge of the ancient reef at 
