1824.J Dr. Berger's Reply to Mr. Henslow. 369 



mical action. Although it is not always easy to distinguish the 

 schistose variety of this rock from argillaceous schistus, it may, 

 however, be effected either by a glass which magnifies consider- 

 ably, or by the blowpipe, which converts the greywacke into 

 a frit only, while the argillaceous schistus fuses ; this is 

 undoubtedly the effect of the fluxes naturally mixed with it. I 

 believe also that the mean specific gravity of this rock is rather 

 greater than that of schistose greywacke. The natural decompo- 

 sition of the latter produces a sterile sandy soil, but little suited to 

 vegetation, on account of the readiness with which water passes 

 through it ; while, on the contrary, it is retained by the plastic 

 property of the decomposed argillaceous schistus. Lastly, the 

 relative positions materially assist the travelling geologist. I 

 had carefully weighed all these distinctive characters at the time 

 of arranging my memoir on the Isle of Man. I have allotted a 

 considerable space to this transition rock, and I find no reason 

 in the supplementary observations of Mr. Henslow, which induces 

 me to alter my opinion in this respect. 



Mr. H. gives so indistinct an idea of the rock which charac- 

 terises what he terms the " quartzose district," that I shall take 

 no further notice of it. 



I am pleased to believe that the circumscription of the lime- 

 stone country is more correctly traced in Mr. Henslow's map 

 than in mine ; but the difference is not assuredly very striking. 

 Without attaching too much value to a conjecture, 1 may inquire 

 of Mr. H. how he supposes the heterogeneous substances 

 which are found in amygdaloids have been formed there.* 



I now arrive at my "great mistake" (p. 494), that of having 

 supposed that the whole extent of the isthmus of Langness was 

 composed of a conglomerate of quartz pebbles loosely attached 

 together. This error on my part may, perhaps, exist, and I 

 thank Mr. H. for having animadverted upon it; \ but I am much 

 disposed to believe, that the rock which, according to Mr. H. 

 forms the greater part of it, ought to be called schistose grey- 

 wacke, rather than argillaceous schistus. 



Mr. Henslow mistakes when he makes me say (p. 495), that 

 all the interval between Scarlet and Poolvash is occupied by a 

 bed of amygdaloidal trap. I mention, on the contrary, the two 

 places in which 1 had observed this rock.]: 



* It is well known that some of these substances, which are of a friable anil delicate 

 nature, often disappear from the paste in which they were inclosed, by the action of 

 atmospheric agents. 



+ I owe it to truth to state, that not having completed the circuit of this isthmus, as 

 my companions may yet remember, 1 had some doubts, which were removed by iVIr. 

 Webster, as to colouring my map uniformly, as has actually been done. But I ought 

 at the same time to remark, that I do not say hi lite text of my memoir, thai tin- iBth- 

 inus of Langness is riitirrly Composed of this conglomerate. I confined myself lo gene- 

 ral statements, which are DOt incorrect. The small extent of tile isthmus of Langness, 

 compared to that of the whole isle, ought, perhaps, to disarm the extreme severity of 

 my critic. 



t " Kaal-Farane and Cromwell's Walk, two places that separate Scarlet Point 

 from the entrance of Poolvash JJay, present an unstiatitied bed of amygdaloid, that OVei- 

 lies die limestone itself." 



New Series, vol. VII. 2 B 



