1821.] Red Rock Mark, or Newer Red Sandstone. 25^ 



a specimen of greenstone as long ago to have attracted notice, 

 and the town of Bovey Tracey stands on a rock of the same 

 nature. 



I forbear to speculate on the probability that the whole extent 

 of the red marie was produced by the degradation of the rocks 

 which have left their fragments still imbedded in its mass. The 

 total absence of those organic remains, which occur so abund- 

 antly in the strata immediately below as well as above, and the 

 general want of consoHdation in its various and heterogeneous 

 beds, certainly argues that its formation took place under dif- 

 ferent circumstances, and by a different process from that of the 

 subjacent slate and limestone, or the superincumbent lias. The 

 strata at DawHshare not everywhere of uniform thickness ; they 

 dip at an angle hardly exceeding 15 to S.E. by S. On this 

 coast they are usually capped by the debris of the green sand 

 formation which covers the neighbouring heights ofHaldon. At 

 Dawlish these debris are much more plentiful than at Teign- 

 mouth. It may be remarked, that while tht^y cover so large a 

 space towards the coast, they are of much scarcer occurrence 

 on the plain of Bovey, which lies under the opposite declivity of 

 Haldon. Some, however, apparently water worn, are found on 

 that tract. I cannot conclude without expressing a wish that 

 the whole extent of this formation were carefully examined by 

 some abler and more instructed observer. 



Article IV. 



A new Method of constructing geometrically/ the Cases of Spheri- 

 cal Triangles by a Developement of their Parts in Piano, By 

 Mr. W. L. Birlibeck. (With a Plate.) 



(To the Editor of the Annals of Philosophy.) 



SIR, . London^ March 10» 



The advantage of constructions as means of checking the 

 calculations of plane trigonometry has been generally so appa- 

 rent, that many trigonometrical writers have given simple geome- 

 trical constructions for the cases of plane triangles. With 

 regard, however, to spherical trigonometry, a different course 

 has been adopted ; and as the consideration of the position of 

 the various circles of the sphere first led to the problem of find- 

 ing certain parts of the triangles formed by their intersections 

 from others that might be given, the writers on this subject have 

 thought it necessary that all these circles should be represented 

 on a plane, as they actually appear on the sphere. Of the 

 several methods of doing this, the stereographic projection has 



r2 



