OF THE NORTH-EAST OF FIFE. 419- 



The north-east extremity of this great belt of volcanic rocks 

 reaches the North Sea at the south side of the Pirth of Tay, and 

 forms, with certain interruptions, the southern shore of the estuary 

 for some 18 or 20 miles ; it here varies from five to ten miles in 

 width, and on its south-eastern edge is in contact with the Upper 

 Old Eed and Carboniferous rocks. It seems probable that the line 

 of contact is a line of fault. 



As may be supposed, the long line of shore in the Firth of Tay 

 furnishes the most convenient section for studying the rocks which 

 compose this igneous belt. Not only are such sections formed by 

 the recent action of the waves of the Firth, but the cliffs of raised 

 sea-beaches in some parts rise one behind the other to the height 

 of two hundred or two hundred and fifty feet, and in many instances 

 these raised clifi's afford sections nearly as good as those presented 

 by the cliffs of the present sea-shore. 



In most of the sections these rocks are, at first sight, very dis- 

 couraging to the student — weathered to one universal dull grey^ 

 greatly altered, crushed and faulted, with patches of tuff, breccias, or 

 amygdaloid insensibly passing into what appears solid lava, and solid 

 lava in the same manner passing into apparently sedimentary rocks, 

 no relation or succession can be easil}- recognized among them. 

 It is therefore not surprising that even able geologists of the last 

 generation simply contented themselves by calling these rocks 

 '^ trap," and so passed them by without further notice. 



In this apparent chaos, however, observation enables us to 

 recognize the remains of order and arrangement. 



In the first place, it is evident that by far the most prevalent 

 rock is altered andesite (porphyrite). So far as I have been able to 

 observe, this rock was erupted from the earliest isolated volcanos of 

 the Lower Old-Eed-Sandstone lake, and continued until the great 

 chain of volcanos, which formed the broad belt of volcanic rocks 

 referred to, had reached their maximum elevation. Of the vast 

 mass of lava and other ejecta thrown out from these volcanos only 

 the most insignificant ruins remain to us. 



The altered andesite is well exhibited in two quarries, about a mile 

 to the south-east of Newport (fig. 1, «, h) ; both of these quarries have 

 been worked for a long time for building-stones and road " metal,'" so 

 that they are cut well back into the hill-side. The uppermost of 

 these quarries (Causewaj^head) has all the appearance of a volcanic 

 " neck," while the lower one (Northfield) is formed of flattened 

 columns considerably bent, so that in Causewaj'head quarry we 

 have probably the vent of an andesite volcano, while that of North- 

 field is cut into a great lava-stream. The rocks of both quarries are 

 very similar in general aspect, only that of Causewayhead is more 

 crystalline than the rock of Northfield, as would be expected if the 

 supposition as to their mode of origin be correct. 



The next most prevalent rock is a highly altered basalt (mela- 

 phyre). The basalt is found almost entirely in the form of dykes 

 and bosses protruding through the andesite, and is consequently 

 more recent than that rock. These basaltic dykes are very numerous 



